The present invention relates generally to fault testing of insulation applied to a cable, wire or other elongated electrical conductor and deals more particularly with an improved insulation testing apparatus of the type wherein an alternating current high test voltage is applied between the insulation and the conductor to reveal holes, voids or other imperfections in the insulation.
Insulated electrical conductors, such as wire or cable, are usually subjected to a high voltage test prior to use as a check against any unseen imperfections which may be present in the insulation. This test may be made either as the insulation is extruded onto the conductor or may be performed in some subsequent operation. Generally, the insulated conductor, the conductor of which is suitably grounded, is passed through an electrode of some type which applies a high voltage to the outer surface of the insulation. Testing apparatus of this type is disclosed in U.S. Pat. Nos. 3,418,570; 3,510,763; 3,514,696; and 4,952,880 issued to Clinton and to which patents reference may be made for a detail description of such testing devices and which disclosures are incorporated herein by reference.
In the AC testing of insulated wire or cable by means of an in-line application of a high test voltage applied between an electrode around the outside of the insulation and the grounded conductor, it is desirable to detect the smallest possible arc from the electrode to the conductor through a pin hole, crevice or other imperfection in the insulation.
Additionally, it is also desirable to prevent the normal corona discharge which occurs in the air spaces between the elements of the electrode and the insulation surface from producing erroneous indications of a fault condition by the detecting means.
A drawback of known testing apparatus is that their designs are such to compromise low current arc sensitivity to achieve a measure of immunity to corona discharge.
A further drawback to known testing apparatus is the sensitivity of the detectors used are degraded by the capacitance in the cable linking the high voltage transformer to the remotely located fault detector.
It is further desirable that the control unit or fault detecting means in such testing apparatus be separate and remotely located from the testing station and the high voltage potential transformer used to excite the electrode by distances of up to 100 meters without degradation of performance of the apparatus.
It is a general aim therefore of the present invention to provide an enhanced sensitivity to low current fault arcs while preventing normal corona discharge from being erroneously detected as a insulation fault.
It is a further aim of the present invention to locate the fault detection means at some distance from the high voltage transformer without altering the detection characteristics of the fault detection means.